Airbags are common used in large vehicles as springs. For example, airbags are used to support a vehicle cab on a chassis to absorb shocks from road bumps and potholes. Airbags are also used to support the bed of a trailer on the frame of the trailer to absorb road shocks. A vehicle manufacturer commonly sets a predetermined height for the airbag. A leveling valve is coupled between cab and frame and maintains the predetermined airbag height by flowing air from a pressured air source into the airbag when the height decreases below a predetermined range (e.g. 4 inches to 5 inches), and to flow air from the airbag into the atmosphere when the height increases beyond the predetermined range. The pressured air source is commonly set to maintain a pressure between 125 and 135 psi, with this pressured air source also being used to operate a horn, pneumatic seat and other miscellaneous equipment on the vehicle. The optimum pressure range for an airbag may be about 40 to 70 psi for a nearly fully loaded (about 75% of maximum design) vehicle (the cab may be always nearly fully loaded).
It is found that when a vehicle with the above leveling system moves over a rough road, that the leveling system "jacks". That is, the airbag height repeatedly increases to a height greater than the design range (4 inch to 5 inches). When the vehicle is at rest for an extended period (e.g. one minute at a stoplight), the optimum height is reached, but then the airbag and chassis jack up again. Then the airbag height increases above the selected range of height, and the airbag pressure has increased significantly, resulting in a considerably harsher ride. Such a harsher ride for the cab of a tractor places greater stress on the driver. A similar condition exists for the primary suspension system of the tractor, where the bag height may be about 15 inches. A system that avoided such jacking, without significantly increasing the cost of the vehicle leveling system, would be of value.